The currently increasing concern about safety and environment facilitates a replacement of traditional flame retardants. For example, halogen based flame retardants such as bromine and chlorine based flame retardants have high flame retardance and afford the advantage that they may be used in so small amounts that compositions loaded therewith may maintain mechanical strength. However, they have a problem of safety in that in case of fire, they generate a substantial volume of halogen gas, by which people within a building may be suffocated and at worst exposed to the risk of death. Phosphate based flame retardants tend to be leached out of the resin surface, posing concerns about toxicity and mutagenesis when discharged into the environment. Red phosphorus based flame retardants have a high level of flame retardance due to a high phosphorus concentration, but generate highly toxic phosphine gas upon incomplete combustion, and red phosphorus itself has the risk of ignition by friction or impacts. Manufacturers refrain from using these traditional halogen, phosphate, and red phosphorus based flame retardants because of potential impacts on safety and environment.
Under these circumstances, it would be desirable to formulate polyurethane to be flame retardant. Polyurethane, which is synthesized by polyaddition reaction between diisocyanate and polyol, has a number of advantages including abrasion resistance, elasticity, weather resistance and solvent resistance, and now find a wide variety of applications. For example, thermoplastic urethane elastomers are used as hoses, tubes, cables, sheets, films, watch bands, and sports shoes. Urethane foams are used as sealants, fillers, and thermal insulators. Recently, the automotive industry started using urethane foams as suspension arm bushings, bump stoppers, engine mounts, bumpers, arm rests, head rests, seat cushions, and the like.
A number of techniques have been proposed for the flame retardation of polyurethane. For example, JP-A H05-70542 and JP-A 2003-003116 disclose urethane resins having phosphorus incorporated in their skeleton using phosphorus-containing polyols. Since the starting reactant, phosphorus-containing polyol is expensive, this method is not accepted in most applications or used in practice.
JP-A 2001-294645 discloses a urethane resin having halogen incorporated in its skeleton using a halogen-containing polyol. However, the starting reactant is expensive, and the use of halogen indicates a lack of safety.
It is also known to add halogen compounds, halogen-phosphorus compounds, halogenated polyethers, halogenated phosphates, and combinations of halogen with antimony trioxide as the filler. This method also lacks safety because halogen is used.
As mentioned above, attempts to render polyurethane resins flame retardant encounter many difficulties. At present, there are available no flame retardant polyurethane resin compositions having sufficient flame retardance to clear a V-0 rating of UL-94 and improved mechanical properties (such as tensile strength and elongation) while overcoming the problems of safety, environment and price.
Citation List                Patent Document 1: JP-A H05-70542        Patent Document 2: JP-A 2003-003116        Patent Document 3: JP-A 2001-294645 (EP 1127908, U.S. Pat. No. 6,706,774)        